Programmer for biostimulator system

ABSTRACT

A biostimulator system comprises one or more implantable devices and an external programmer configured for communicating with the implantable device or devices via bidirectional communication pathways comprising a receiving pathway that decodes information encoded on stimulation pulses generated by ones of the implantable device or devices and conducted through body tissue to the external programmer.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.11/549,605, filed Oct. 13, 2006, now U.S. Pat. No. 7,945,333; whichapplication claims the benefit of priority to and incorporates herein byreference in their entirety for all purposes, U.S. ProvisionalApplication Nos. 60/726,706 entitled “LEADLESS CARDIAC PACEMAKER WITHCONDUCTED COMMUNICATION,” filed Oct. 14, 2005; 60/761,531 entitled“LEADLESS CARDIAC PACEMAKER DELIVERY SYSTEM,” filed Jan. 24, 2006;60/729,671 entitled “LEADLESS CARDIAC PACEMAKER TRIGGERED BY CONDUCTEDCOMMUNICATION,” filed Oct. 24, 2005; 60/737,296 entitled “SYSTEM OFLEADLESS CARDIAC PACEMAKERS WITH CONDUCTED COMMUNICATION,” filed Nov.16, 2005; 60/739,901 entitled “LEADLESS CARDIAC PACEMAKERS WITHCONDUCTED COMMUNICATION FOR USE WITH AN IMPLANTABLECARDIOVERTER-DEFIBRILLATOR,” filed Nov. 26, 2005; 60/749,017 entitled“LEADLESS CARDIAC PACEMAKER WITH CONDUCTED COMMUNICATION AND RATERESPONSIVE PACING,” filed Dec. 10, 2005; and 60/761,740 entitled“PROGRAMMER FOR A SYSTEM OF LEADLESS CARDIAC PACEMAKERS WITH CONDUCTEDCOMMUNICATION,” filed Jan. 24, 2006; all by Peter M. Jacobson.

BACKGROUND

Cardiac pacing electrically stimulates the heart when the heart'snatural pacemaker and/or conduction system fails to provide synchronizedatrial and ventricular contractions at appropriate rates and intervalsfor a patient's needs. Such bradycardia pacing provides relief fromsymptoms and even life support for hundreds of thousands of patients.Cardiac pacing may also give electrical overdrive stimulation intendedto suppress or convert tachyarrhythmias, again supplying relief fromsymptoms and preventing or terminating arrhythmias that could lead tosudden cardiac death.

Cardiac pacing is usually performed by a pulse generator implantedsubcutaneously or sub-muscularly in or near a patient's pectoral region.The generator usually connects to the proximal end of one or moreimplanted leads, the distal end of which contains one or more electrodesfor positioning adjacent to the inside or outside wall of a cardiacchamber. The leads have an insulated electrical conductor or conductorsfor connecting the pulse generator to electrodes in the heart. Suchelectrode leads typically have lengths of 50 to 70 centimeters.

Known pulse generators can include various sensors for estimatingmetabolic demand, to enable an increase in pacing rate proportional andappropriate for the level of exercise. The function is usually known asrate-responsive pacing. For example, an accelerometer can measure bodymotion and indicate activity level. A pressure transducer in the heartcan sense the timing between opening and closing of various cardiacvalves, or can give a measure of intracardiac pressure directly, both ofwhich change with changing stroke volume. Stroke volume increases withincreased activity level. A temperature sensor can detect changes in apatient's blood temperature, which varies based on activity level. Thepacemaker can increase rate proportional to a detected increase inactivity.

Pulse generator parameters are usually interrogated and modified by aprogramming device outside the body, via a loosely-coupled transformerwith one inductance within the body and another outside, or viaelectromagnetic radiation with one antenna within the body and anotheroutside.

Although more than five hundred thousand pacemakers are implantedannually, various well-known difficulties are present.

The pulse generator, when located subcutaneously, presents a bulge inthe skin that patients can find unsightly or unpleasant. Patients canmanipulate or “twiddle” the device. Even without persistent twiddling,subcutaneous pulse generators can exhibit erosion, extrusion, infection,and disconnection, insulation damage, or conductor breakage at the wireleads. Although sub-muscular or abdominal placement can address some ofconcerns, such placement involves a more difficult surgical procedurefor implantation and adjustment, which can prolong patient recovery.

A conventional pulse generator, whether pectoral or abdominal, has aninterface for connection to and disconnection from the electrode leadsthat carry signals to and from the heart. Usually at least one maleconnector molding has at least one terminal pin at the proximal end ofthe electrode lead. The at least one male connector mates with at leastone corresponding female connector molding and terminal block within theconnector molding at the pulse generator. Usually a setscrew is threadedin at least one terminal block per electrode lead to secure theconnection electrically and mechanically. One or more O-rings usuallyare also supplied to help maintain electrical isolation between theconnector moldings. A setscrew cap or slotted cover is typicallyincluded to provide electrical insulation of the setscrew. The complexconnection between connectors and leads provides multiple opportunitiesfor malfunction.

For example, failure to introduce the lead pin completely into theterminal block can prevent proper connection between the generator andelectrode.

Failure to insert a screwdriver correctly through the setscrew slot,causing damage to the slot and subsequent insulation failure.

Failure to engage the screwdriver correctly in the setscrew can causedamage to the setscrew and preventing proper connection.

Failure to tighten the setscrew adequately also can prevent properconnection between the generator and electrode, however over-tighteningof the setscrew can cause damage to the setscrew, terminal block, orlead pin, and prevent disconnection if necessary for maintenance.

Fluid leakage between the lead and generator connector moldings, or atthe setscrew cover, can prevent proper electrical isolation.

Insulation or conductor breakage at a mechanical stress concentrationpoint where the lead leaves the generator can also cause failure.

Inadvertent mechanical damage to the attachment of the connector moldingto the generator can result in leakage or even detachment of themolding.

Inadvertent mechanical damage to the attachment of the connector moldingto the lead body, or of the terminal pin to the lead conductor, canresult in leakage, an open-circuit condition, or even detachment of theterminal pin and/or molding.

The lead body can be cut inadvertently during surgery by a tool, or cutafter surgery by repeated stress on a ligature used to hold the leadbody in position. Repeated movement for hundreds of millions of cardiaccycles can cause lead conductor breakage or insulation damage anywherealong the lead body.

Although leads are available commercially in various lengths, in someconditions excess lead length in a patient exists and is to be managed.Usually the excess lead is coiled near the pulse generator. Repeatedabrasion between the lead body and the generator due to lead coiling canresult in insulation damage to the lead.

Friction of the lead against the clavicle and the first rib, known assubclavian crush, can result in damage to the lead.

In many applications, such as dual-chamber pacing, multiple leads can beimplanted in the same patient and sometimes in the same vessel. Abrasionbetween the leads for hundreds of millions of cardiac cycles can causeinsulation breakdown or even conductor failure.

Data stored in memory of implanted pulse generators is typically madeavailable to a physician or other personnel for collection and/oranalysis. For example, information is sought regarding systemperformance and trouble-shooting relating to the device, lead system,and/or patient in an acute, clinical setting. The information isgenerally supplied via a telemetry capability between the externalprogrammer and the implanted device. In addition, an external programmercan be used to adjust parameters of multi-function implantable medicaldevices, such as pacing rate, pulse amplitude, sensed signal gain, andpulse timing and coordination.

Typically, an external programmer used during a telemetry procedure ispositioned remotely from the patient. A programming head of theprogrammer such as a wand or other external device, containing anantenna or coil, is connected to the remainder of the programmer via astretchable coil cable and is positioned over the patient's implanteddevice site for programming or telemetry interrogation of the implanteddevice.

Communication between the implanted medical device and the externalprogrammer is facilitated by receiving and transmitting circuitryincluded within the implanted medical device and external programmer.Bandwidth is generally kept low to minimize power consumed by theimplanted medical device. Power consumption is a consideration indesigning implantable medical devices since the devices are typicallypowered by a depletable energy source, such as a primary battery.Replacement of an implanted medical device due to battery depletion canbe costly and inconvenient.

Therefore, minimization of power consumption by the implanted medicaldevice is a design and operational consideration. To facilitate powerconsumption management, transmitter and receiver circuitry can bepowered down when not in use but are to be awakened when desired toenable communication. Awakening can occur periodically, in which theimplantable device checks for a communication signal at regularintervals. The awakening process can otherwise be achieved by usingelectromagnetic energy coupled to the receiving antenna or coil tofacilitate the wake up function. Awakening techniques result in acomplicated telemetry protocol, which generally results in longer linkuptimes. In addition, the awakening techniques employ a relatively largeantenna or coil, which is undesirable and inconsistent with a physicallycompact implanted medical device.

In addition to power reduction and small size, another design criterionfor implanted medical devices is accurate communication of data.Communication often occurs in environments such as hospitals anddoctors' offices, which can be noisy due to the presence of otherelectronic and electromagnetic sources. To achieve robustness of thelink, bandwidth is generally kept low, with small packet sizes. Toassure that data are transmitted accurately, the antenna or coil in theimplantable device is typically positioned to maximize signal strength,both transmitted and received.

SUMMARY

According to an embodiment of a biostimulator system, one or moreimplantable devices and an external programmer are configured forcommunicating with the implantable device or devices via bidirectionalcommunication pathways comprising a receiving pathway that decodesinformation encoded on stimulation pulses generated by ones of theimplantable device or devices and conducted through body tissue to theexternal programmer.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention relating to both structure and method ofoperation may best be understood by referring to the followingdescription and accompanying drawings, in which similar referencecharacters denote similar elements throughout the several views:

FIGS. 1A and 1B are pictorial diagrams showing embodiments ofbiostimulator systems comprising two leadless cardiac pacemakers securedto internal and to exterior surfaces of the heart, respectively, and anexternal programmer and two surface electrodes;

FIG. 2 is a schematic block diagram depicting an embodiment of anexternal programmer that can be used in a biostimulator system andadapted to communicate via conductive techniques;

FIG. 3 is a time waveform graph showing a sample of modulatedcommunication transmitted from an external programmer to a system of oneor more leadless cardiac pacemakers;

FIG. 4 is a time waveform graph illustrating a conventional pacingpulse;

FIG. 5 is a time waveform graph depicting a pacing pulse adapted forcommunication as implemented for an embodiment of the illustrativepacing system;

FIG. 6 is a time waveform graph showing a sample series of pacing pulsesin which information is encoded by altering the interval betweenindividual pulses; and

FIGS. 7A-7E are schematic flow charts depicting techniques that can beused in various embodiments of methods for communicating in animplantable biostimulator system.

DETAILED DESCRIPTION

An external programmer can be used with a system of one or more leadlesscardiac pacemakers. Individual leadless cardiac pacemakers can beimplanted adjacent to the inside or outside wall of a cardiac chamber.The programmer uses a minimum of two electrodes in electrical contactwith the skin to communicate with each pacemaker through conduction.Information is passed from programmer to implant through a modulationtechnique designed to avoid stimulation of skeletal muscles.Communication from implant to programmer is performed by encodinginformation on the pacing pulses.

The programmer includes a user interface to display status and settingsinformation for one or more individual implantable pacemakers, andenables the user to change programmable parameters on an individualimplantable pacemaker. The programmer also can display theelectrocardiogram sensed from the same two external electrodes on theskin. The programmer can perform tasks including electrocardiogramsensing, retrieving status information from implantable pacemakers, andchanging configuration parameters of the implantable pacemakerssimultaneously through the same set of electrodes.

Use of conducted communication of information improves over standardmethods of communication in several aspects. For example, theillustrative conductive techniques enable communication withoutrequiring a programmer head to be held undesirably close to the patientor to be held in a precise position relative to the implant site for anextended period of time. The illustrative conductive communication alsoenables power consumption to be reduced due to substantially lowercurrent requirements and eliminating peak power demands currentlyimposed by existing inductive and radio frequency (RF) systems. Also,the conductive communication technique uses elements generally alreadyexisting in the implanted pulse generator, such as the therapeuticelectrodes that function as an input-output device, enabling eliminationof a coil or antenna that are conventionally used for communication andreducing complexity and component count significantly.

Referring to FIGS. 1A and 1B, schematic pictorial views depictembodiments of biostimulator systems 100A, 100B that communicate viaconductive communication. The biostimulator systems 100A, 100B compriseone or more implantable devices 102 and an external programmer 104configured for communicating with the one or more implantable devices102 via bidirectional communication pathways comprising a receivingpathway that decodes information encoded on stimulation pulses generatedby one or more of the implantable devices 102 and conducted through bodytissue to the external programmer 104.

According to the illustrative arrangement, the bidirectionalcommunication pathways can be configured for communication with multipleleadless cardiac pacemakers 102 via two or more electrodes 106 andconduction through body tissue.

In accordance with various biostimulator system embodiments, an externaldevice or module 104 is connected by a communication transmissionchannel and has transmitting and receiving functional elements for abidirectional exchange of information with one or more implanted medicaldevices 102. The communication channel includes two or more electrodes106 which can be affixed or secured to the surface of the skin. From thepoint of the skin, the communication transmission channel is wireless,includes the ion medium of the intra- and extra-cellular body liquids,and enables electrolytic-galvanic coupling between the surfaceelectrodes and the implantable modules 104.

In the biostimulator systems 100A, 100B, the bidirectional communicationpathways can further comprise a transmitting pathway that passesinformation from the external programmer 104 to one or more of theimplantable devices 102 by direct conduction through the body tissue bymodulation that avoids skeletal muscle stimulation using modulatedsignals at a frequency in a range from approximately 10 kHz to 100 kHz.

Information transmitted from the external programmer 104 to theimplanted devices 102 is conveyed by modulated signals at theapproximate range of 10 kHz to 100 kHz which is a medium-high frequency.The signals are passed through the communication transmission channel bydirect conduction. A modulated signal in the frequency range has asufficiently high frequency to avoid any depolarization within theliving body which would lead to activation of the skeletal muscles anddiscomfort to the patient. The frequency is also low enough to avoidcausing problems with radiation, crosstalk, and excessive attenuation bybody tissue. Thus, information may be communicated at any time, withoutregard to the heart cycle or other bodily processes. No restriction isimposed regarding location of electrode placement on the body becauselow signal attenuation enables the signal to travel throughout the bodyand to be received by the implanted devices 102.

In some embodiments, the bidirectional communication pathways canfurther comprise a receiving pathway including a low-pass filter adaptedto separate the electrocardiogram from the information signals. The samesurface electrodes 106 that are used to transmit the information throughthe communication channel may also be used to detect a patient'selectrocardiogram. Electrocardiogram frequencies are generally between 1and 100 Hz, far lower than the 10 kHz to 100 kHz range of frequenciesused to transmit information through the communication transmissionchannel. Therefore, the electrocardiogram can be separated from theinformation signal by a low-pass filter and can optionally be displayedby the programmer 104. In addition to low-pass filtering, blankingtechniques that are typical in processing of cardiac signals can be usedwhen the communication channel is active to prevent noise or erroneoussignals from the communication channel affecting the electrocardiogramchannel.

Because a plurality of implantable devices 102 can be present,communication of information from the programmer is detected by alldevices, enabling information to be sent to each implanted devicewithout sending the same information multiple times.

In various embodiments and applications, the bidirectional communicationpathways can further comprise a transmitting pathway that passesinformation from the programmer 104 to the one or more implantabledevices 102 in a common communication event whereby information is sentto one or more target devices of the implantable devices 102 using aselected technique. For example, information specific to a singleimplantable device or a subset of implantable devices having a uniqueaddress can be assigned to the single implantable device or the subsetof implantable devices and encoded in the information. In anothertechnique, information can designate a specific function that isexecuted by a particular implantable device or a particular subset ofimplantable devices. The information is passed to one or moreimplantable devices without sending individual address information foractivating execution by the particular implantable device or theparticular subset of implantable devices alone. In another technique,information can designate a specific function that is executed by aparticular implantable device or a particular subset of implantabledevices that have programming specific to the function adapted torecognize the received information is relevant to the function.

Specifically, information that is specific to a single implanted deviceor a subset of devices can be sent. A unique address can be assigned toeach device or subset. The address can be encoded in the informationsent to the plurality of devices, and any individual device can make useonly of information that matches either the address or the address ofthe subset to which the particular device belongs.

In another technique, if each implanted device 102 or subset of devices102 serves a specific function, which is different from other implanteddevices, then information may be passed to the specific device or subsetwithout the additional overhead of a group or individual address. Forexample, the device or subset can be responsible for only a specificfunction. When the programmer transmits information to the entire group,but the information is relevant to only the device or subset of thatgroup, then any devices that cannot make use of the information mayignore the information. Each device has unique programming specific to aparticular function and can recognize whether received information isrelevant to the function. Devices operative in conjunction with thetechnique can be non-generic and perform specific functions, or can begeneric devices with general functionality that can be made morespecific by programming. Accordingly, functionality of a device can bedefined at manufacture or may be defined at implantation or thereafter.The function of each device can be defined at the time of manufactureand the devices labeled or marked such that the associated function canbe known upon inspection.

In some embodiments, the one or more implantable devices 102 cancomprise one or more leadless cardiac pacemakers that generate cardiacpacing pulses and encode information onto the generated cardiac pacingpulses by selective alteration of pacing pulse morphology that is benignto therapeutic effect and energy cost of the pacing pulse. The cardiacpacing pulses conduct into body tissue via the electrodes forantenna-less and telemetry coil-less communication. For informationtransmitted from the implanted leadless cardiac pacemaker 102 to theexternal programmer 104, a communication scheme can be used in which theinformation is encoded on the pacing pulse. The pulse morphology isaltered to contain the encoded information without altering thetherapeutic benefits of the pacing pulse. The energy delivered by thepacing pulse remains essentially the same after the information isencoded. The external programmer 104 receives the pacing pulses throughthe associated surface electrodes 106. Encoded information is drawn fromthe pacing pulses and can contain state information of the implantableleadless cardiac pacemaker, such as battery voltage, lead impedance,sensed electrocardiogram amplitude, pacemaker current drain, programmedparameters, or other parameters.

The leadless cardiac pacemaker or pacemakers 102 can be configured todetect a natural cardiac depolarization, time a selected delay interval,and deliver an information-encoded pulse during a refractory periodfollowing the natural cardiac depolarization. By encoding information ina pacing pulse, power consumed for transmitting information is notsignificantly greater than the power used for pacing. Information can betransmitted through the communication channel with no separate antennaor telemetry coil. Communication bandwidth is low with only a smallnumber of bits encoded on each pulse.

In some embodiments, information can be encoded using a technique ofgating the pacing pulse for very short periods of time at specificpoints in the pacing pulse. During the gated sections of the pulse, nocurrent flows through the electrodes of a leadless cardiac pacemaker.Timing of the gated sections can be used to encode information. Thespecific length of a gated segment depends on the programmer's abilityto detect the gated section. A certain amount of smoothing or low-passfiltering of the signal can be expected from capacitance inherent in theelectrode/skin interface of the programmer as well as theelectrode/tissue interface of the leadless cardiac pacemaker. A gatedsegment is set sufficiently long in duration to enable accuratedetection by the programmer 104, limiting the amount of information thatcan be transmitted during a single pacing pulse. Accordingly, atechnique for communication can comprise generating stimulation pulseson stimulating electrodes of an implanted biostimulator and encodinginformation onto generated stimulation pulses. Encoding information ontothe pulses can comprise gating the stimulation pulses for selecteddurations at selected timed sections in the stimulation pulses wherebygating removes current flow through the stimulating electrodes andtiming of the gated sections encodes the information.

Another method of encoding information on pacing pulses involves varyingthe timing between consecutive pacing pulses in a pulse sequence. Pacingpulses, unless inhibited or triggered, occur at predetermined intervals.The interval between any two pulses can be varied slightly to impartinformation on the pulse series. The amount of information, in bits, isdetermined by the time resolution of the pulse shift. The steps of pulseshifting are generally on the order of microseconds. Shifting pulses byup to several milliseconds does not have an effect on the pacing therapyand cannot be sensed by the patient, yet significant information can betransmitted by varying pulse intervals within the microsecond range. Themethod of encoding information in variation of pulses is less effectiveif many of the pulses are inhibited or triggered. Accordingly, atechnique for communication can comprise generating stimulation pulseson stimulating electrodes of an implanted biostimulator and encodinginformation onto generated stimulation pulses comprising selectivelyvarying timing between consecutive stimulation pulses.

Alternatively or in addition to encoding information in gated sectionsand/or pulse interval, overall pacing pulse width can be used to encodeinformation.

The three described methods of encoding information on pacing pulses canuse the programmer 104 to distinguish pacing pulses from the patient'snormal electrocardiogram, for example by recognition of the specificmorphology of the pacing pulse compared to the R-wave generated duringthe cardiac cycle. For example, the external programmer 104 can beadapted to distinguish a generated cardiac pacing pulse from a naturalcardiac depolarization in an electrocardiogram by performing comparativepattern recognition of a pacing pulse and an R-wave produced during acardiac cycle.

The illustrative external programmer 104 and associated operatingmethods or techniques enable presentation to the user of informationgathered from the implanted biostimulator or leadless cardiac pacemakers102 using conductive communication. Some of the information to bepresented may include battery voltage, lead impedance, electrocardiogramamplitude, or current drain of the device. The information can bepresented in addition to other information such as parameters to be setand programmed into the leadless cardiac pacemaker. The information canbe presented to a user on a display screen. Some embodiments orconfigurations of an external programmer 104 can include a secondarylink, for example either wireless or through a cable, to another displaydevice, such as a handheld computer or terminal. The secondary link canalso include communication over a local area network or the internet fordisplay at a remote terminal.

FIG. 1A depicts a sample configuration involving the external programmer104 and two endocardially implanted leadless cardiac pacemakers 102. Theexternal programmer 104 is physically connected to the skin surface viatwo electrodes 106, which serve three functions. First, the electrodes106 transmit encoded information from the programmer 104 to theimplanted leadless cardiac pacemakers 102 using a modulated signal at amedium frequency 10 kHz to 100 kHz. Second, the electrodes 106 receiveinformation from individual leadless cardiac pacemakers 102 by detectingencoded information in the pacing pulses of the leadless cardiacpacemakers 102. Third, the electrodes 106 receive surfaceelectrocardiogram for display and analysis by the programmer 104.

In FIG. 1A, the two leadless cardiac pacemakers 102 are implantedendocardially. Thus, in a biostimulator system 100A or 100B animplantable device 102 may comprise one or more leadless cardiacpacemakers that can be implanted adjacent to an inside or an outsidewall of a cardiac chamber. Referring to FIG. 1B, a similar system isrepresented with a difference that the two leadless cardiac pacemakers102 are implanted by affixing to the exterior surface of the heart. Theelectrodes 106 and programmer 104 function similarly in arrangementsshown in FIGS. 1A and 1B whether the leadless cardiac pacemakers 102 areimplanted endocardially or epicardially (on the external heart surface).No restriction is imposed that the leadless cardiac pacemakers are allimplanted inside or all implanted outside the heart. One or more may beimplanted endocardially along with others implanted on the outer surfaceof the heart. The functioning of the programmer 104 is substantially thesame. Although two electrodes 106 are shown in FIGS. 1A and 1B, two isgenerally the minimum number for adequate conductive communication. Moreelectrodes can be used, enabling an electrocardiogram (ECG) to be sensedat multiple vectors for better analysis. More than two electrodes alsoenable a choice of vectors for conducted communication with the leadlesscardiac pacemakers, thereby maximizing the signal to noise ratio of thesystem. FIGS. 1A and 1B each depict two leadless cardiac pacemakers 102.One, two, or more leadless cardiac pacemakers can be implanted,depending on the number of pacemakers appropriate for effective therapy.

In various embodiments, the external programmer 104 can be configured toperform one or more operations such as electrocardiogram sensing,retrieving status information from implanted pacemakers, modifyingconfiguration parameters of multiple implanted pacemakers simultaneouslyin information passed through a common electrode set, displayingelectrocardiograms, displaying information received from the at leastone implantable device, and others.

In various embodiments, a pacemaker 102 can manage power consumption todraw limited power from an internal battery, thereby reducing devicevolume. Each circuit in the pacemaker can be designed to avoid largepeak currents. For example, cardiac pacing can be achieved bydischarging a tank capacitor (not shown) across the pacing electrodes.Recharging of the tank capacitor is typically controlled by a chargepump circuit. In a particular embodiment, the charge pump circuit can bethrottled to recharge the tank capacitor at constant power from thebattery. The one or more leadless cardiac pacemakers can be configuredto charge the tank capacitor in preparation for stimulation pulsegeneration, time one or more windows between pulse generation, disablecharging of the tank capacitor during the one or more timed windows, andenable a receive amplifier in the implanted biostimulator while the tankcapacitor is disabled.

In some embodiments, the external programmer 104 can detect astimulation pulse from a leadless cardiac pacemaker 102 and transmitdata after a selected delay to coincide with a window that the leadlesscardiac pacemaker's receiving amplifier is enabled.

The implantable devices 102 can encode and/or decode information usingvarious techniques such as encoding the information using pacing pulsewidth, binary-coded notches in a pacing pulse, modulation of off-timebetween pacing pulses, or other suitable encoding techniques. Theexternal programmer 104 can encode and/or decode information usingon-off keying encoding and modulation techniques depicted in FIG. 3.However, any other appropriate method can be used whereby a modulatedbit-stream can be generated at a medium high frequency, for examplefrequency-shift keying, frequency modulation, or amplitude shift keying.

Referring to FIG. 2, a schematic block diagram shows an embodiment of anexternal programmer 204 adapted for communicating with an implantedbiostimulator system using conducted communication. The externalprogrammer 204 comprises an interface 208 configured for coupling to atleast two electrodes 206 that make electrical contact with body skin forcommunicating with one or more implanted biostimulators. The externalprogrammer 204 further comprises bidirectional communication pathways210R and 210T coupled to the interface 208 and configured forbidirectional communication with the one or more implantedbiostimulators. The communication pathways comprise a receiving pathway210R that decodes information encoded on stimulation pulses generated bythe one or more implanted biostimulators and conducted through bodytissue.

The bidirectional communication pathways 210R and 210T are configuredfor communication with one or more leadless cardiac pacemakers via theelectrodes 206 and conduction through body tissue.

The external programmer 204 can have bidirectional communicationpathways 210R and 210T that further comprise a transmitting pathway 210Tthat passes information from the programmer 204 to one or more implantedbiostimulators by conduction through the body tissue using modulationthat avoids skeletal muscle stimulation.

In some arrangements, the bidirectional communication pathways 210R and210T can be further specified to comprise a transmitting pathway thatpasses information from the programmer 204 to the one or more implantedbiostimulators by direct conduction using modulated signals at afrequency in a range from approximately 10 kHz to 100 kHz. Also in somearrangements, the two or more electrodes 206 and the bidirectionalcommunication pathways 210R and 210T can be configured for bidirectionalinformation signal communication and for sensing an electrocardiogram.

Also in some embodiments, the bidirectional communication pathways 210Rand 210T can further comprise a transmitting pathway 210T that passesinformation from the programmer 204 to multiple implanted devices in acommon communication event. In some embodiments or selected operatingconditions, the transmitting pathway 210T can be arranged to passinformation from the programmer 204 to multiple implanted devices in acommon communication event whereby information specific to a singleimplanted device or a subset of implanted devices have a unique addressassigned to the single implanted device or the subset of implanteddevices and encoded in the information. The transmitting pathway 210Tcan also be arranged to pass information from the programmer 204 tomultiple implanted devices in a common communication event wherebyinformation designates a specific function that is executed by aparticular implanted device or a particular subset of implanted devices.The information is passed to the multiple implanted devices withoutindividual address information for activating execution by theparticular implanted device or the particular subset of implanteddevices alone. The transmitting pathway 210T can also be arranged,either alone or in combination with other techniques, to passinformation from the programmer 204 to multiple implanted devices in acommon communication event whereby information designates a specificfunction that is executed by a particular implanted device or aparticular subset of implanted devices that comprise programmingspecific to the function adapted to recognize the received informationis relevant to the function.

In the illustrative embodiment, the bidirectional communication pathways210R and 210T comprise the two or more electrodes 206 forming aconductive communication path between the programmer 204 and the skinsurface, and a transmitting pathway 210T. The transmitting pathway 210Tcomprises a processor 212, a command/message encoder 230, a modulator232, and an amplifier 236. The processor 212 is configured tocommunicate information to one or more implanted leadless cardiacpacemakers. The command/message encoder 230 is coupled to the processor212 via a parallel interface and configured to encode and serialize datainto a bit stream. Information encoding can be selected from encodingtechniques such as on-off keying, frequency-shift keying, frequencymodulation, and amplitude shift keying. The modulator 232 is coupled tothe command/message encoder 230 and receives and modulates theserialized data using a frequency in a range from approximately 10 kHzto approximately 100 kHz. The amplifier 236 is coupled to the modulator232 and increases signal amplitude to a level suitable for robustconducted communication.

The bidirectional communication pathways 210R and 210T further comprisea receiving pathway 210R including a low-pass filter 214 adapted toseparate the electrocardiogram from the information signals.

In various embodiments and arrangements, the bidirectional communicationpathways 210R and 210T further comprise a receiving pathway 210R thatreceives information at the programmer 204 from the one or moreimplanted biostimulators by conduction through the body tissue. Thereceiving pathway 210R can decode information, for example by decodingdata that is encoded by the biostimulators using pacing pulse width,using binary-coded notches in a pacing pulse, using modulation ofoff-time between pacing pulses, or other suitable techniques forencoding data in the biostimulators.

In the illustrative embodiment, the bidirectional communication pathways210R and 210T couple to the two or more electrodes 206 forming aconductive communication path between the programmer 204 and the skinsurface, and a receiving pathway 210R. The receiving pathway 210Rcomprises an electrocardiogram (ECG) amplifier/filter 214, ananalog-to-digital converter (ADC) which is not shown in FIG. 2, and theprocessor 212. The electrocardiogram (ECG) amplifier/filter 214 includesa differential band-pass amplifier configured to select and amplifysignals in a frequency range from approximately 1 Hz to approximately100 Hz. The analog-to-digital converter (ADC) is configured to digitizethe filtered and amplified signal. The processor 212 is coupled to theADC and configured to receive and optionally display ECG data, andconfigured to decode information encoded into cardiac pacing pulses.

The programmer 204 may further comprise a processor 212 coupled to thebidirectional communication pathways and configured to managecommunication with one or more biostimulators, for example leadlesscardiac pacemakers. Leadless cardiac pacemakers can be implantedadjacent to an inside or an outside wall of a cardiac chamber asdepicted in FIGS. 1A and 1B.

As depicted in FIG. 2, external electrodes 206 enable a conductivecommunication path between the programmer 204 and the skin surface.Electrocardiogram (ECG) signals enter an ECG amplifier/filter 214, whichcan include a differential band-pass amplifier. In general, an ECGsignal has spectral components in a range between 1 Hz and 100 Hz.Band-pass filter poles for the ECG amplifier/filter 214 can be selectedsuch that sufficient signal energy is passed within the 1 Hz to 100 Hzrange, while filtering other signals that are not associated withcardiac activity. The ECG signal can be amplified and digitized using ananalog-to-digital converter (ADC). Once digitized, the signal is passedto the processor, for example central processing unit (CPU) 212.

In some embodiments, the electrodes 206 can be implemented with morethan two electrodes to enable an electrocardiogram (ECG) to be sensed atmultiple vectors and further to enable selection from among the multiplevectors for conducted communication with implanted leadless cardiacpacemakers so that system signal-to-noise ratio can be improved ormaximized.

The CPU 212 receives and optionally displays ECG data using a displayinterface 216 and can also display other data acquired from theimplanted leadless cardiac pacemaker acquired through the encoded pacingpulses, such as battery voltage, lead impedance, sensed cardiac signalamplitude, or other system status information. The CPU 212 also canaccept input from a user via a keyboard and/or touch-screen interface218. Some examples of user input are selected pacing rate or pacingpulse amplitude for implanted leadless cardiac pacemakers. The CPU 212can also communicate over a network interface 220 to other data entry ordisplay units, such as a handheld computer or laptop/desktop unit. Thenetwork interface 220 can be cabled or wireless and can also enablecommunication to a local area network or the internet for greaterconnectivity.

The processor 212 is coupled to the bidirectional communication pathwaysand configured to perform one or more of various operations such aselectrocardiogram sensing, retrieving status information from implantedpacemakers, modifying configuration parameters of multiple implantedpacemakers within a single or multiple cardiac cycles in informationpassed through a common electrode set, and other operations. A displayinterface 216 coupled to the processor 212 can be configured to displayan electrocardiogram sensed from the electrodes 206. In somearrangements or embodiments, a secondary link 220 can be coupled to theprocessor 212 and configured for unidirectional or bidirectionalwireless or cable transmission to and/or from a remote display and/ordata-entry device to display an electrocardiogram sensed from the atleast two electrodes, and/or to control the programmer and/or at leastone implanted biostimulator.

The CPU 212 can execute operations based on firmware stored innon-volatile memory (Flash) 222. The non-volatile memory 222 can also beused to store parameters or values that are to be maintained when poweris removed. The CPU 212 uses volatile memory or random access memory(RAM) 224 as general storage for information such as ECG data, statusinformation, swap memory, and other data. A battery and supply regulator226 gives a constant voltage supply to the programmer 204 during normaloperation. A clock module 228 generates a system clock signal used bythe CPU 212 and by interface blocks for timing.

The CPU 212, during operation to communicate information to one or moreimplanted leadless cardiac pacemakers, sends the information over aparallel interface to a command/message encoder 230, which serializesthe data into a bit stream. Serialized data is sent to a modulator 232.The serialized bit-stream is modulated, for example using a frequencybetween 10 kHz and 100 kHz. An optional separate modulator clock 234supplies a timing signal at a selected carrier frequency that may beused by the modulator 232. An amplifier 236 sets signal amplitude to alevel that enables robust conducted communication. A sample of amodulated bit-steam is shown in FIG. 3 wherein logic high is shown as amedium high frequency sine wave. An encoding and modulation techniquedepicted in FIG. 3 is on-off keying. However, any other appropriatemethod whereby a modulated bit-stream can be generated at a medium highfrequency may be used, for example frequency shift keying, frequencymodulation, or amplitude shift keying.

Because multiple biostimulator devices can be implanted, communicationof information from the programmer 204 can be detected by all devices,enabling information to be sent to each implanted device without sendingthe same information multiple times.

If information for communication is specific to a single implanteddevice or a subset of devices, a unique address can be assigned to eachdevice or subset. The address is encoded in the information sent to theplurality of devices, and any individual device can have the option tomake use of information that either matches the address or the addressof the subset to which the particular device belongs.

If each implanted device or a subset of devices performs a specificfunction which is different from other implanted devices, theninformation can be passed to the specific device or subset without theadditional overhead of a group or individual address. For example, whenthe device or subset is responsible for only a specific function. Whenthe programmer 204 transmits information to the entire group, but theinformation is relevant to only the device or subset of that group, thenany devices that cannot make use of the information may ignore theinformation as superfluous. The technique presumes that each device haveunique programming specific to the associated function, and each devicehave capability to recognize whether or not received information isrelevant to the function. Devices using the illustrative technique arenot generic. The function of each device can be defined at the time ofmanufacture or at the time of implant or thereafter. The devices arelabeled or marked such that the associated function can be known uponinspection.

To reduce the peak current for operation of the leadless cardiacpacemakers, a technique can be used in which a window or multiplewindows occur between subsequent pacing pulses during which the leadlesscardiac pacemaker does not charge pacing tank capacitor in preparationfor the next pacing pulse. Instead the pacemaker enables an internalreceiving amplifier. Because the programmer 204 can sense pacing pulsesfrom the implanted devices, the programmer 204 can time datatransmission to coincide with the pre-defined synchronous window orwindows. A reduced peak current capability occurs because the chargerand receiving amplifier, both power intensive elements, never have to beoperated together. Because the data transmission is generally very shortcompared to the period between pacing pulses, the window techniqueshould not significantly lower the ability of the leadless cardiacpacemaker to charge the pacing tank capacitor effectively between pacingpulses.

Referring again to FIG. 2, data acquired by the programmer 204 from aspecific implanted leadless cardiac pacemaker is received at the surfaceelectrodes 206 and passes to an amplifier/filter 240, which functions toremove noise from the incoming signal. Any filtering performed by theamplifier/filter 240 is designed to leave encoded pulses intact as muchas possible. A message decoder 238 determines whether the receivedsignal is actually a pacing pulse or another signal, such as a cardiacR-wave.

FIG. 4 shows a sample pacing pulse, for example a typical output-pulsewaveform for a conventional pacemaker. The approximately-exponentialdecay is due to discharge of a capacitor in the pacemaker through theapproximately-resistive load presented by the electrodes/tissueinterface and leads. Typically the generator output is capacitor-coupledto one electrode to ensure net charge balance. The pulse duration isshown as T0 and is typically 500 microseconds. When the pacemaker 102 issupplying a pacing pulse but is not sending data for communication, thewaveform can resemble that shown in FIG. 4.

To encode data on the pacing pulse, specific portions of the pulse aregated. Timing of the gated segments defines the specific data carried bythe pacing pulse. FIG. 5 is a time waveform graph showing a sampleoutput-pulse waveform of the illustrative pacemaker that communicatessignals using conduction during a time when the pacemaker is sendingdata for communication and also delivering a pacing pulse.

FIG. 5 shows that the pulse generator 102 has divided the output pulseinto shorter pulses 501, 502, 503, 504; separated by notches 505, 506,and 507. The pulse generator 102 times the notches 505, 506, and 507 tofall in timing windows W1, W2, and W4 designated 508, 509, and 511respectively. Note that the pacemaker 102 does not form a notch intiming window W3 designated 510. The timing windows are each shownseparated by a time T1, approximately 100 microseconds in the example.

As controlled by a processor in a leadless cardiac pacemaker 102, apulse generator in the pacemaker selectively generates or does notgenerate a notch in each timing window 508, 509, 510, and 511 so thatthe device 102 encodes four bits of information in the pacing pulse. Asimilar scheme with more or fewer timing windows can send more or fewerbits per pacing pulse. The width of the notches is small, for exampleapproximately 15 microseconds, so that the delivered charge and overallpulse width, specifically the sum of the widths of the shorter pulses,in the pacing pulse is substantially unchanged from that shown in FIG.4. Accordingly, the pulse shown in FIG. 5 can have approximately thesame pacing effectiveness as that shown in FIG. 4, according to the lawof Lapique which is well known in the art of electrical stimulation.

In a leadless cardiac pacemaker, a technique can be used to conservepower when detecting information carried on pacing pulses from otherimplanted devices. The leadless cardiac pacemaker can have a receivingamplifier that implements multiple gain settings and uses a low-gainsetting for normal operation. The low-gain setting could beinsufficiently sensitive to decode gated information on a pacing pulseaccurately but could detect whether the pacing pulse is present. If anedge of a pacing pulse is detected during low-gain operation, theamplifier can be switched quickly to the high-gain setting, enabling thedetailed encoded data to be detected and decoded accurately. Once thepacing pulse has ended, the receiving amplifier can be set back to thelow-gain setting. For usage in the decoding operation, the receivingamplifier is configured to shift to the more accurate high-gain settingquickly when activated. Encoded data can be placed at the end of thepacing pulse to allow a maximum amount of time to invoke the high-gainsetting.

As an alternative or in addition to using notches in the stimulationpulse, the pulses can be generated with varying off-times, specificallytimes between pulses during which no stimulation occurs. The variationof off-times can be small, for example less than 10 milliseconds total,and can impart information based on the difference between a specificpulse's off-time and a preprogrammed off-time based on desired heartrate. For example, the device can impart four bits of information witheach pulse by defining 16 off-times centered on the preprogrammedoff-time. FIG. 6 is a graph showing a sample pulse generator outputwhich incorporates a varying off-time scheme. In the figure, time T_(P)represents the preprogrammed pulse timing. Time T_(d) is the delta timeassociated with a single bit resolution for the data sent by the pulsegenerator. The number of T_(d) time increments before or after themoment specified by T_(P) gives the specific data element transmitted.The receiver of the pulse generator's communication has advanceinformation of the time T_(P). The communication scheme is primarilyapplicable to overdrive pacing in which time T_(P) is not dynamicallychanging or altered based on detected beats.

FIG. 6 shows the technique of conveying information by modulating theoff-time between pacing pulses. Alternatively or in addition to the twoillustrative coding schemes, overall pacing pulse width can be used toimpart information. For example, a paced atrial beat may exhibit a pulsewidth of 500 microseconds and an intrinsic atrial contraction can beidentified by reducing the pulse width by 30 microseconds. Informationcan be encoded by the absolute pacing pulse width or relative shift inpulse width. Variations in pacing pulse width can be relatively smalland have no impact on pacing effectiveness.

The illustrative example avoids usage of radiofrequency (RF)communication to send pacing instructions to remote electrodes on abeat-to-beat basis to cause the remote electrodes to emit a pacingpulse. RF communication involves use of an antenna andmodulation/demodulation unit in the remote electrode, which increaseimplant size significantly. Also, communication of pacing instructionson a beat-to-beat basis increases power requirements for the main bodyand the remote electrode. In contrast, the illustrative system andstimulator do not require beat-to-beat communication with anycontrolling main body.

The illustrative leadless pacemaker 102 includes an internal powersource that can supply all energy for operations and pulse generation.In contrast, some conventional implanted pulse generators have remotepacing electrodes that receive some or all energy from an energy sourcethrough an RF induction technique, an energy transfer scheme thatemploys a large loop antenna on the remote electrode which increasessize significantly. In addition, energy transfer with the RF inductiontechnique is inefficient and is associated with a significant increasein battery size of the energy source. In contrast, the illustrativeleadless pacemaker 102 uses an internal battery and does not requireenergy to be drawn from outside sources. Also in the conventionalsystem, the energy source receives sensing information by RFcommunication from the remote electrodes and sends pacing instructionsto the electrodes on a beat-to-beat basis in a configuration that usesan addressing scheme in which the identity of specific remote pacingelectrodes is stored in the energy source memory. The conventionalmethod can also be inefficient due to overhead for transmitting anidentification number from/to a generic pacing electrode at implantand/or during sensing. The illustrative leadless pacemaker 102 avoidssuch overhead through a structure in which pulse generationfunctionality is independent within a single implantable body.

Another conventional technology uses a system of addressable remoteelectrodes that stimulate body tissue without requiring a main body tosend commands for individual stimulations. The remote electrodes arespecified to be of a size and shape suitable for injection rather thanfor endocardial implantation. A controller sets operating parameters andsends the parameters to remote electrodes by addressable communication,enabling the remote electrodes function relatively autonomously whileincurring some overhead to controller operations. However, the remoteelectrodes do not sense or monitor cardiac information and rely on themain body to provide sensing functionality. In contrast, theillustrative leadless pacemaker 102 combines pacing and sensing ofintrinsic cardiac activity in a single implantable body.

To ensure the leadless cardiac pacemaker functions correctly, a specificminimum internal supply voltage is maintained. When pacing tankcapacitor charging occurs, the supply voltage can drop from apre-charging level which can become more significant when the batterynears an end-of-life condition and has reduced current sourcingcapability. Therefore, a leadless cardiac pacemaker can be constructedwith a capability to stop charging the pacing tank capacitor when thesupply voltage drops below a specified level. When charging ceases, thesupply voltage returns to the value prior to the beginning of tankcapacitor charging.

In another technique, the charge current can be lowered to prevent thesupply voltage from dropping below the specified level. However,lowering the charge current can create difficulty in ensuring pacingrate or pacing pulse amplitude are maintained, since the lower chargecurrent can extend the time for the pacing tank capacitor to reach atarget voltage level.

Schemes can be implemented for transmitting data from the implant to theprogrammer that do not significantly increase the current consumption ofthe pacemaker. For example, the pacemaker could transmit datacontinuously in a loop, with no consumption penalty.

The method of encoding data using modulation of off-time between pacingpulses is less effective if pulses are inhibited, since data can betransmitted using only pacing pulses generated by the pacemaker. Whendata are encoded in binary-coded notches in the pacing pulse or byvarying pacing pulse width, if a therapeutic pacing pulse is inhibited,then the leadless cardiac pacemaker can still generate a non-therapeuticpulse during the refractory period of the heart after the sensed beat,although the pacing pulse has the sole purpose of transmitting data tothe programmer or optionally to at least one other implantedbiostimulator.

Referring to FIGS. 7A-7E, schematic flow charts depict techniques thatcan be used in various embodiments of methods for communicating in animplantable biostimulator system. According to FIG. 7A, an illustrativemethod 700 comprises monitoring 702, at an external programmer,electrical signals conducted through body tissue to body surfaceelectrodes and detecting 704 pulses generated by a body-implantedbiostimulator. The external pacemaker decodes 706 information encodedinto the generated pulse by the body-implanted biostimulator.

Referring to FIG. 7B, a method 710 can further comprise generating 712cardiac pacing pulses at an implanted leadless cardiac pacemaker.Information is encoded 714 onto generated cardiac pacing pulses at theimplanted leadless cardiac pacemaker by selective alteration of pacingpulse morphology that is benign to therapeutic effect and energy cost ofthe pacing pulse. In various embodiments, the implanted leadless cardiacpacemaker can encode the information using one or more techniques suchas encoding using pacing pulse width, using binary-coded notches in apacing pulse, and using modulation of off-time between pacing pulses.The cardiac pacing pulses are conducted 716 into body tissue viaelectrodes for antenna-less and telemetry coil-less communication. Theinformation encoded onto generated cardiac pacing pulses can includepacemaker state information, battery voltage, lead impedance, sensedcardiac signal amplitude, pacemaker current drain, programmedparameters, and the like.

Referring to FIG. 7C, a method 720 can further comprise generating 722cardiac pacing pulses at an implanted leadless cardiac pacemaker andencoding 724 information onto generated cardiac pacing pulses at theimplanted leadless cardiac pacemaker by selective alteration of pacingpulse morphology that is benign to therapeutic effect and energy cost ofthe pacing pulse. The implanted leadless cardiac pacemaker detects 726 anatural cardiac depolarization and inhibits 728 cardiac pacing pulsedelivery with delay for delivery during a refractory period followingthe natural cardiac depolarization. The cardiac pacing pulses areconducted 730 into body tissue via electrodes for antenna-less andtelemetry coil-less communication.

Referring to FIG. 7D, various embodiments of a method 740 can comprisecharging 742 a tank capacitor in preparation for stimulation pulsegeneration. Stimulation pulses are generated 744 on stimulatingelectrodes of an implanted biostimulator and information encoded 746onto generated stimulation pulses. One or more windows can be timed 748between pulse generations. Charging of the tank capacitor is disabled750 during the one or more timed windows and a receiving amplifier inthe implanted biostimulator is enabled 752 while the tank capacitor isdisabled. The external programmer senses 754 the stimulation pulsesgenerated by the implanted biostimulator and transmits 756 informationfrom the external programmer to the implanted biostimulator to coincidewith the one or more timed windows. For example, the external programmercan detect a stimulation pulse from the implanted biostimulator, time aselected delay interval, and transmit data after the selected delay tocoincide with a window that the implanted biostimulator's receiveamplifier is enabled.

Referring to FIG. 7E, various embodiments of a method 760 can comprisephysically connecting 762 the external programmer to a body surface viatwo or more body surface electrodes and communicating 764 informationamong the external programmer and one or more implanted leadless cardiacpacemakers. Encoded information is transmitted 766 from the externalprogrammer to the implanted leadless cardiac pacemakers via the bodysurface electrodes using a modulated signal at a frequency in a range ofapproximately 10 kHz to approximately 100 kHz. The external programmerreceives 768 the information via the body surface electrodes from one ormore of the implanted leadless cardiac pacemakers by detectinginformation encoded into generated pacing pulses. The externalprogrammer can also receive 770 a surface electrocardiogram via the bodysurface electrodes for display and analysis.

Terms “substantially”, “essentially”, or “approximately”, that may beused herein, relate to an industry-accepted tolerance to thecorresponding term. Such an industry-accepted tolerance ranges from lessthan one percent to twenty percent and corresponds to, but is notlimited to, component values, integrated circuit process variations,temperature variations, rise and fall times, and/or thermal noise. Theterm “coupled”, as may be used herein, includes direct coupling andindirect coupling via another component, element, circuit, or modulewhere, for indirect coupling, the intervening component, element,circuit, or module does not modify the information of a signal but mayadjust its current level, voltage level, and/or power level. Inferredcoupling, for example where one element is coupled to another element byinference, includes direct and indirect coupling between two elements inthe same manner as “coupled”.

While the present disclosure describes various embodiments, theseembodiments are to be understood as illustrative and do not limit theclaim scope. Many variations, modifications, additions and improvementsof the described embodiments are possible. For example, those havingordinary skill in the art will readily implement the steps necessary toprovide the structures and methods disclosed herein, and will understandthat the process parameters, materials, and dimensions are given by wayof example only. The parameters, materials, and dimensions can be variedto achieve the desired structure as well as modifications, which arewithin the scope of the claims. Variations and modifications of theembodiments disclosed herein may also be made while remaining within thescope of the following claims. Phraseology and terminology employedherein are for the purpose of the description and should not be regardedas limiting. With respect to the description, optimum dimensionalrelationships for the component parts are to include variations in size,materials, shape, form, function and manner of operation, assembly anduse that are deemed readily apparent and obvious to one of ordinaryskill in the art and all equivalent relationships to those illustratedin the drawings and described in the specification are intended to beencompassed by the present description. Therefore, the foregoing isconsidered as illustrative only of the principles of structure andoperation. Numerous modifications and changes will readily occur tothose of ordinary skill in the art whereby the scope is not limited tothe exact construction and operation shown and described, andaccordingly, all suitable modifications and equivalents may be included.

1. A biostimulator system for a patient comprising: at least oneimplantable device configured to generate modulated signals and toencode information on the modulated signals; and an external programmercomprising at least two electrodes and bidirectional communicationpathways configured to couple with the electrodes, the externalprogrammer being configured to be located outside the patient andcommunicate with the at least one implantable device through the atleast two electrodes of the external programmer via the bidirectionalcommunication pathways, the bidirectional communication pathwayscomprising a receiving pathway configured to decode information encodedon modulated signals generated by the at least one implantable deviceand conducted through body tissue to the at least two electrodes of theexternal programmer.
 2. The system of claim 1 wherein the bidirectionalcommunication pathways further comprise a transmitting pathwayconfigured to pass information from the at least two electrodes of theexternal programmer to the at least one implantable device by directconduction through the body tissue.
 3. The system of claim 2 wherein themodulated signals are transmitted at a frequency configured to avoidskeletal muscle stimulation.
 4. The system of claim 1 wherein thebidirectional communication pathways further comprise a transmittingpathway configured to pass information from the at least two electrodesof the external programmer to the at least one implantable device in acommon communication event, whereby information is sent to at least onetarget device of the at least one implantable device.
 5. The system ofclaim 4 wherein the transmitting pathway is further configured to passinformation specific to a single implantable device or a subset ofimplantable devices having a unique address assigned to the singleimplantable device or the subset of implantable devices.
 6. The systemof claim 4 wherein the transmitting pathway is further configured topass information designating a specific function that is to be executedby a particular implantable device or a particular subset of implantabledevices without passing individual address information for activatingexecution by the particular implantable device or the particular subsetof implantable devices alone.
 7. The system of claim 4 wherein thetransmitting pathway is configured to pass information designating aspecific function that is to be executed by the at least one implantabledevice, the at least one implantable device comprising programmingspecific to the function and being further adapted to recognize that thereceived information is relevant to the function.
 8. The system of claim1 further wherein the receiving pathway further comprises a low-passfilter adapted to separate an electrocardiogram from the informationsignals encoded on the modulated signals.
 9. The system of claim 8further comprising a controller configured to blank noise and unwantedsignals from the electrocardiogram.
 10. The system of claim 1 whereinthe at least one implantable device comprises at least one leadlesscardiac pacemaker.
 11. The system of claim 10 wherein the at least oneleadless cardiac pacemaker is configured to be implanted adjacent to aninside or an outside wall of a cardiac chamber.
 12. The system of claim10 wherein the at least one leadless cardiac pacemaker is configured todeliver an information-encoded pulse during a refractory periodfollowing a natural cardiac depolarization.
 13. The system of claim 10wherein the at least one leadless cardiac pacemaker is configured tocharge a tank capacitor in preparation for stimulation pulse generation,time one or more windows between pulse generation, disable charging ofthe tank capacitor during the one or more timed windows, and enable areceive amplifier while the tank capacitor is disabled.
 14. The systemaccording to claim 13 wherein the external programmer is furtherconfigured to detect a stimulation pulse from the at least one leadlesscardiac pacemaker and transmit data after a selected delay to coincidewith a window within which the receive amplifier of the at least oneleadless cardiac pacemaker is enabled.